The invention relates to an improved incandescent lamp comprising a bulb of soft glass having a filling gas containing a halogen constituent, together with the inert gas, in which the halogen constituents of the filling gas do not react with the alkali ions of the glass bulb. The invention also relates to a method for manufacturing such a lamp.
In known halogen cycle incandescent lamps the possibility of a reaction between the halogen constituent of the filling gas and the alkali ions (which are components of many glasses) was avoided because the lamp bulb was manufactured from quartz or hard glass which both contain either no or only minor proportions of alkali ions. Such glasses require high processing temperatures. It was desired to manufacture halogen cycle incandescent lamps having bulbs comprising soft glass.
The hardness of the glass is determined by the linear thermal coefficient of expansion or the socalled transformation temperature, respectively. Glasses containing alkali for instance soda lime glass, have a greater thermal expansion than glasses without alkali content and are called "soft glasses". It is an advantage of these glasses that they may be processed more easily than quartz glasses and hard glasses.
When soft glass bulbs are used for halogen cycle incandescent lamps there arises the difficulty that the halogen of the filling gas reacts with the alkali ions, more particularly with the sodium ions of the bulb glass to yield a thermally stable compound, for instance in accordance with the reaction Na.sub.2 SiO.sub.3 +2HBr.revreaction.SiO.sub.2 +H.sub.2 O+2NaBr. The effective halogen content of the filling gas is reduced. As a result the tungsten halogen cycle process which characterizes the halogen cycle incandescent lamp can no longer be maintained which leads to lamp blackening. In addition, the reaction products of the alkali and halogen cause undesired deposits in the lamp bulb.
To prevent the reaction of the halogen constituents of the filling gas with the alkali constituents of the bulb glass, it has already been proposed in the U.S. Pat. No. 4,256,988 to apply to the inner surface of the bulb and the surfaces of the interior components a coating of a continuous impregnating layer which consists substantially of a metal oxide (Al.sub.2 O.sub.3). Such halogen cycle incandescent lamps are not manufactured at the present time. Possibly the technique for manufacturing these lamps has not yet matured. Possibly, there is a difficulty in applying a sufficiently dense protective layer so that no alkali ions may diffuse through it without the optical quality of the glass bulb being impaired.